add -P switch to df to fix GetDirectoryRootInLinux

If the Filesystem is too long it will wrap to the next line which causes
an IndexOutOfRangeException when attempting to get the volume name.

Added the -P switch which will output it POSIX style and all on one
line.
Merge pull request #45 from ascjones/dev

fixed IndexOutOfRangeException in GetDirectoryRootInLinux: add -P switch to df to avoid line wrap